These conversion devices are usually produced within integrated circuits. They may be continuous time (or CT according to the term commonly used by those skilled in the art) or else discrete time (or DT according to the term commonly used by those skilled in the art).
Usually, this conversion is carried out with the aid of a sigma delta modulator because it provides a good resolution of conversion and rejects quantization noise outside the payload band of the signal.
FIG. 1 illustrates schematically an example of a structure usually used for sigma delta modulators within a continuous time converter CT. The structure of sigma delta modulators is usually based on the combination of an integrator and a summer, the assembly being looped. The diagram of a sigma delta modulator is therefore that of a closed-loop control system.
The sigma delta modulator MSD comprises at the head a summer SOM (subtractor) receiving an analogue input signal x(t) having a frequency Fe. This subtractor is in this instance followed by an integrator INT the output of which is connected to the input of a quantization means QTZ (sampler) the output of which forms the output of the modulator. The output of the quantization means QTZ is looped back to the negative input of the summer SOM by means of a digital/analogue converter DAC. The quantization means QTZ converts the signal from the integrator INT into a digital signal y(n). Moreover, the modulator MSD comprises a clock generator CLK in order to generate a sampling frequency Fs intended for the quantization means QTZ and for the converter DAC.
The “delta” modulation is based on the quantization of the modification of the analogue input signal x(t). The presence of a “sigma” integrator in the modulator gives the modulator the title of “sigma delta” modulator. The integrator usually comprises a low-pass filter
This being so, there are also bandpass sigma delta modulators which comprise, instead of an integrator, a bandpass filter. These modulators, although they have no integrator, still retain, the title “sigma delta modulator”.
In practice, in radio-frequency applications, use is made of a bandpass filter and a sampling frequency Fs which is approximately equal to four times the frequency of the analogue input signal x(t). Since the sampling frequency is very high, this modulator MSD does not make it possible to convert high-frequency analogue signals; usually the frequency of the input signals is limited to a few gigahertz.
Moreover, as indicated above, in order to convert the analogue signals, it is also possible to use a discrete time analogue/digital converter DT. These converters also use sigma delta modulators but unlike the continuous-time use described above, these modulators receive a sampled signal as an input. These converters also comprise a first frequency mixer situated upstream of the modulator in order on the one hand to sample the analogue input signal and on the other hand to transpose the frequency of the input signal around a carrier frequency. Then the modulator converts the sampled signal received into a digital output signal. This converter also comprises a second mixer situated at the output of the modulator in order to again transpose the frequency of the digital output signal around the frequency of the input signal.
Sampling the analogue input signal makes it possible to use a sampling frequency value for the sigma delta modulator that is lower than that used in the continuous time converter CT described above. Specifically, for a discrete time converter DT, the sampling frequency is approximately equal to twice the frequency of the analogue input signal.
These discrete time converters DT do not make it possible to convert the analogue signals at frequencies higher than the continuous time converters CT. Moreover, they have a complex architecture because they use two frequency mixers.